The present invention relates generally to an ultrasonic phased array transducer and more particularly to an acoustic composite material used with the ultrasonic phased array and a method for making.
A typical ultrasonic phased array transducer used in medical and industrial applications includes one or more piezoelectric elements placed between a pair of electrodes. The electrodes are connected to a voltage source. When a voltage is applied, the piezoelectric elements are excited at a frequency corresponding to the applied voltage. As a result, the piezoelectric elements emit an ultrasonic beam of energy into a media that it is coupled to at frequencies corresponding to the convolution of the transducer's electrical/acoustical transfer function and the excitation pulse. Conversely, when an echo of the ultrasonic beam strikes the piezoelectric elements, each element produces a corresponding voltage across its electrodes.
In addition, the ultrasonic phased array transducer typically includes an acoustic backing layer (i.e., a backfill) coupled to the piezoelectric elements. The backfill has a low impedance in order to direct the ultrasonic beam towards a patient or object. Typically, the backfill is made from a lossy material that provides high attenuation for diminishing reverberations. Also, the ultrasonic phased array includes acoustic matching layers coupled to the piezoelectric elements opposite from the backfill layer. The acoustic matching layers transform the acoustic impedance of the patient or object under inspection to a value closer to that of the piezoelectric elements. This improves the efficiency of sound transmission to the patient/object and increases the bandwidth over which sound energy is transmitted.
A problem associated with conventional matching layers is that they must be made from materials having impedances ranging from about 2 MRayls to about 12 MRayls. For optimal matching, the thickness and acoustic impedance of the matching layers are typically determined by using transducer design models. Frequently, the transducer design models require certain material parameters for which there are no materials available. If these materials are not available, then composite materials are typically used or a design compromise is made which sacrifices bandwidth and/or sensitivity. Examples of acoustic composite materials are particles suspended in a matrix (i.e., a 0-3 material) and engineered silicon materials with a "bed of nails" structure (i.e., a 1-3 connectivity). The particles suspended in a matrix approach provides a controlled impedance, but suffers from high attenuation and inhomogeneity resulting from the random distribution of particles in the matrix. The silicon "bed of nails" approach provides a controlled impedance and homogeneity, but requires an expensive and lengthy fabrication process. Thus, there is a need for an acoustic material that provides controlled impedance and low attenuation.